Ever been writing merrily in Microsoft Word, only to get a green "grammar-error" squiggle under either “that” or “which”? Why does that happen? After all, when you begin a phrase with “that” or “which,” it almost seems like they can be used interchangeably.
She wore a funny hat that was purple.
She wore a funny hat which was purple.
While these both tend to fly in conversation, one of these examples contains a grammatical error—and if you guessed the second one, you’re right. The only way for the second example to be error-free (in other words, the only way to abolish that ugly green squiggle) is to add a comma before “which”—like so:
She wore a funny hat, which was purple.
So, why does it matter whether we choose “that” or “which”?
First, let me explain what a restrictive clause is:
A restrictive clause (also known as an essential clause) is an adjective clause—in other words, a descriptive sentence element—that provides information that is essential to the meaning of the sentence. Restrictive clauses often start with the words that, who, whom, or whose (though for simplicity’s sake, this blog post will just focus on “that”).
To demonstrate, I’ve highlighted the restrictive clauses in the examples below:
I love the book that you gave me.
What is the name of the store that you recommended last week?
The one that’s over there on the left.
Without the highlighted clauses, the sentences above would lose an essential piece of information and be vague and kinda useless.
A non-restrictive clause (also known as a non-essential clause) is an adjective clause that contains non-essential information; essentially, these clauses could be left out and the meaning of the sentence would remain intact. Non-restrictive clauses simply add bonus information. Most often they start with the word which, though they can also start with who, whom, or whose (we’re keepin' it simple with the most common, “which”).
The final difference between restrictive and non-restrictive clauses is the comma, which always offsets a non-restrictive clause.
See what I did there?!
Her favourite flowers were tulips, which she found to be very beautiful.
The vacation was over, which made him feel sad.
The cat, which was hiding under a pile of clothes, remained undetected.
Without these highlighted clauses, the sentences would be rather basic, but they would still make sense.
LET'S WRAP THIS BABY UP.
So to figure out whether you start your adjective clause with “that” or “which,” first ask yourself:
Is this phrase essential to the meaning of my sentence?
Restrictive / non-restrictive clauses and the use of “that” or “which” are a finicky editorial detail that your editor will bear with grace, but we hope we’ve helped you lose a few green squiggles and earn a few more points with your readers and publishers.